1. Field of the Invention
This invention relates to a split connecting rod with a hole formed parallel to the separation plane and leading from the wall away from the separating notch and to a method of producing such a connecting rod by fracture separation.
2. Description of Related Art
The U.S. Pat. No. 4,569,109 to Fetouh discloses split bearing arrangements of connecting rods formed by means of fracture separation (cracking) which involves splitting off a bearing cap and defining the bearing bore. Through the use of this fracture-separation method, an accurately fitting joint between the bearing base and the bearing cap is achieved along the fracture surfaces, so that an accurate fit of the fine-machined bearing portion of the connecting rod is ensured with the associated pin of the crankshaft.
In order to reduce the fracture separation forces needed to form the bearing cap, a sintered connecting rod part may be used as a blank, which requires lower fracture forces on account of the material brittleness, as described in EP 0 800 009. Alternatively, as described in DE 43 34 555, with cast bearing blanks the fracture point may be specifically embrittled by embedding a brittle material. However, both methods lead to weaken the entire workpiece and are therefore not suitable for the production of bearing arrangements on connecting rods which are subjected to a high operating load and high forces and therefore impose especially high demands on the material strength. An example of this high strength requirement comes from the commercial-vehicle sector of motor-vehicle construction where connecting rods typically require strengths preferably obtainable by forged parts made of steel alloys. Here the material composition is selected in such a way that, on the one hand, high toughness is ensured, but, on the other hand, the admixing of a high carbon proportion increases the brittleness and thus improves the cracking capacity. For a specific fracture separation, a sharp V-shaped separating notch is made in the connecting rod in the region of the desired fracture separation plane, preferably in the region of the bearing surface. For the fracturing operation, this separating notch serves as a starting point for the starting crack. During the fracturing operation, the separation surfaces produced are predetermined and of annular configuration between the outside surface of the rod defined by the rod eye""s cross-sectional shape and the inside surface formed by the usual through-holes provided for bolts used to subsequently attach the rod""s bearing cap to its bearing saddle.
However, such an annular predetermined separation creates a problem relating to alignment or meeting of the fracture surfaces. One separation surface starts from the separating notch located on the inner wall of the connecting-rod eye, then branches into separate surfaces as the fracture encircles the through-hole in opposite directions. If the two surfaces do not meet at the same point at the opposite side from the notch a double fractures occurs which in an extreme case can cause material chipping which can lead to a defective missing spot thus weakening the material in the annular separation region. Depending on the geometry of the predetermined separation surface, up to 10% to 15% of the parts can be affected by such defects after cracking; and the connecting rods then no longer meet the strength requirements imposed on them and must be scraped. To lesson the problem, the cross section of the fracture plane, and thus also of the web at which the double fractures normally occurs, can be made smaller. However, this has an adverse effect on the stability and strength of the overall connecting rod.
The abovementioned double fracture problem is avoided, for example, in forged connecting rods made according to U.S. Pat. No. 3,994,054 to Cudden-Fletcher. In this patent, a through-hole is provided which serves as a point of application for a tapered cracking mandrel positioned on the connecting rods in the region of each predetermined separation surface parallel to the fracture plane. The mandrel divides the separation region into two (in each case cohesive) parts. In this patent, however, the inner walls of the through-holes are provided with separating notches which on one hand is very expensive to do and on the one hand requires a relatively large diameter of the through-holes. This leads to weakening of the connecting rod in the fracture surface region.
An object of the invention is to improve the process of fracture separating a connecting rod in such a way that the entire connecting rod has the highest possible strength and can be produced with the least possible production outlay and results in the least possible scrapage. In particular, the object of the invention is to configure the predetermined required separation region in such a way that perfectly accurately fitting joint surfaces as large as possible are produced while at the same time the occurrences of double fractures are minimised.
This object is achieved according to the invention by the features described hereinafter in detail. Accordingly, in the region of each predetermined separation surface of the connecting rod, a tubular hole is formed parallel to the predetermined separation surface and which extends from the through-hole to the outer surface or wall of the connecting-rod eye. The tubular hole leads into the wall of the connecting-rod eye opposite to the area of the separating notch. As the previously described two branches of the separation plane encircle the through-hole and approach each other from opposite directions, the tubular hole catches the dual fracture surfaces and thus prevents the occurrence of double or misaligned cracks.
An advantage of the configuration of the connecting rod according to the invention therefore results in a pronounced reduction in the scrapage during the cracking process. In this case, the tubular hole diameter may be selected to be very small, so that the area of the fracture surface is little reduced by the incorporation of the tubular hole and thus the stability and strength of the connecting rod is not reduced. The tubular hole is expediently arranged diametrically opposite the separating notch, so that both branches of the fracture surface essentially cover an equal length path before meeting at the tubular hole. Furthermore, the formation of a second tubular hole extending from the through-hole to the separating notch has been proved to be desirable by facilitating the division of the separation surface into the two separate individual surfaces which are progress separately from the notch and around the through-hole. These tubular holes are expediently produced during the machining phase by drilling and therefore require only a small additional working step.
In a preferred embodiment, the separating notch required for initiating the starting crack during the fracture separation of the connecting-rod eye is formed by broaching. Broaching has a production advantage in that it can be carried out during the course of the other machining. Alternatively, the separating notch may be made with the aid of laser machining; this method can ensure that the fracture runs in an exactly reproducible manner, a factor which leads to increased process reliability of the cracking process.